Add "Native" option for QWKnet call-out cmd-lines

This really shouldn't be necessary to toggle (at least now) on Windows, since we treat all off-line executions as "native", but for *nix, it could make a big difference if trying to invoke a native program or shell script for a QWKnet call-out and it would either fail due to no DOS support or try to executing using Linux-DOSEMU (unless the command's program name was in the global "native program list" configured in SCFG->External Programs).